Block and tackle window balances have become popular because of their compact size and ease of installation. They combine a system of pulleys with an extension spring to convert high spring tension applied over a short working distance to a lower spring tension applied over a longer working distance. The extension spring and pulley system are arranged within a rigid balance channel, with the extension spring anchored at one end of the balance channel and the pulley system anchored at the other end. In most block and tackle balances, the balance channel is mounted in the jamb of the window frame; and a cord, which is reeved through the pulley system, is attached to a sash shoe that slides in the jamb with the sash. The extension spring and pulley system are sized so that a desired lifting force is applied to the window sash throughout the entire range of sash travel within the window frame. A disadvantage of this type of balance is that the movement of the sash is limited by the presence of the balance in the jamb. In some cases, the travel is limited so much that the lower sash of the open window blocks egress through the window in escaping a fire.
To solve the problem of limited sash movement, the balance can be mounted upside down in the window sash with the balance channel attached to the sash shoe and the cord attached to the window jamb or frame. However, prior art window balances of this kind tend to be susceptible to contamination from dirt and dust, especially when mounted upside down. Particles work their way between the pulley bores and the pulley axles, increasing friction and wear. Thus, prior art block and tackle window balances are not as durable or reliable as is desired.
Some prior art block and tackle window balances that are less susceptible to contamination require the use of bushings and other parts. This is disadvantageous in that the use of additional parts increases the complexity of the machines and the likelihood of their failure. Also, the additional parts increase the cost of manufacture and assembly of the window balances.